The present invention relates broadly to impedance standards and in particular to an impedance standard apparatus for calibrating microwave impedance instrumentation.
In the prior art, impedance can be measured in transmission lines by analysis of the behavior of electric fields propagating along their axes. In an idealized coaxial line or waveguide with lossless, unity dielectric-constant insulation and perfectly conducting walls, a wave will propagate along the axis without alteration so long as the cross-sectional dimensions remain uniform. The relationship between the electric and magnetic fields will remain constant; and because they are respectively proportional to voltage and current, the ratio of these fields will define an impedance for the line.
When the wave reaches the end of the transmission line, a portion determined by the impedance discontinuity at that point is reflected backwards toward the source. The incident and reflected waves then add together to form a stationary interference pattern, or standing wave, that has maxima and minima occurring alternately at intervals of a quarter wavelength. The maxima occur at those points at which the waves reinforce each other and measure the sum of the two amplitudes; the minima, conversely, measure their difference. The ratio of maximum to minimum voltage is called the voltage-standing-wave ratio (VSWR).
The present invention provides a standard impedance apparatus by which such measurements may be made quickly and accurately.